1. Field of the Invention
Embodiments of the invention provide a method for standardising the size of a focal spot of an X-ray tube, and find particularly advantageous but non-exclusive application in the field of medical imaging. However, embodiments of the invention can also be applied to other fields in which radiography or radiology examinations are made.
Embodiments of the invention are configured to control, optimise and standardise the size of the focal spot of an X-ray tube.
Embodiments of the invention may standardise a contrast modulation function in an image produced.
Embodiments of the invention may also keep the quality of the images produced constant in all the fields of operation of the X-ray tube.
Embodiments of the invention may also increase the service life or operating life of an X-ray tube.
Embodiments of the invention also relate to X-ray tubes configured to operate using a method described herein.
2. Description of the Prior Art
An X-ray tube, mounted in a medical radiology apparatus, comprises a cathode and an anode. The cathode and the anode are enclosed in a vacuum-tight envelope in order to set up electrical insulation between these two electrodes.
The cathode has a tungsten filament housed in a metal part with a shape appropriate to playing the role of an electronic lens, known as a concentration element or concentrator. The cathode produces an electron beam that is received by the anode on a small surface representing a focal spot from which the X-rays are emitted.
The filament is made incandescent by the flow of a current given by a generator connected to the terminals of the filament. When a positive voltage of some kilovolts relative to the cathode is applied to the anode, electrons emitted by the filament are accelerated towards the anode by the electrical field and bombard the anode or anti-cathode on a surface known as a focal spot. The focal spot thus becomes the main source of emission of X-radiation. The X-radiation is produced throughout the zone situated in front of the anti-cathode except for the grazing incidences.
At present, the image quality in radiology is linked, inter alia, to the dimensions of the focal spot of the X-ray tube. Studies on the sharpness and contrast factors of a radiology image are based on the known concept of the contrast modulation function. The studies clearly show that the greater the size of the focal spot, the greater the extent to which the image of an object is marred by fuzziness. This fuzziness is chiefly due to the superimposition of a large number of images coming from points constituted by the surface of the focal spot, since this focal spot itself is not a pinpoint spot.
In the prior art, to reduce the size of the focal spot, a negative, fixed bias voltage is applied to the concentrator. The greater the absolute value of this bias voltage, the greater the reduction of the size of the focal spot to an optimum value. This fixed bias voltage is determined for a given electron current of the tube or X-ray flow rate.
However, these X-ray tubes have drawbacks. During the operation of the X-ray tube, the shape of the energy profile of the focal spot varies. Consequently, the size of the focal spot and as well as the contrast modulation function varies substantially with the power supply voltage and the intensity of the electron current. This variation is inherent to the medical application, depending on the region to be examined. The variation of the contrast modulation function may impair the quality of the images produced.
This impairment may thus limit the resolution of a radiography image obtained from the focal spot, making it more difficult to detect small-sized microcalcifications for example, in the case of mammography.
The bias voltage applied to the tube is fixed and determined for a power supply voltage of the tube and/or for a given X-ray flow rate so as to adjust the size of the focal spot. The bias voltage is therefore no longer efficient when the power supply voltage and/or the electron current of the tube vary, giving rise to a variation in the size of the focal spot and, potentially, a reduction of the sharpness and contrast of the radiography image.
Furthermore, under the effect of the electron bombardment, the major part of the energy of the electrons incident to the focusing zone on the anode is converted into calorific energy. The heat thus collects on the anode which heats up. When there is a variation in the power supply voltage and the electron current of the tube, the associated variation in the size of the focal spot may lead to excessive energy being obtained. This excessive energy may damage the surface of said focal spot.